It is a confrontation as passionate as anything seen between Javert and Jean Valjean, pitting Cameron Mackintosh against the orchestra of Les Misérables, one of the theatre impresario's biggest successes.

Mackintosh confirmed yesterday that the members of the show's current orchestra – some of whom have played with Les Misérables since it opened 25 years ago – were being forced to reapply for their jobs. The move follows plans for an expanded orchestra with three additional members to accommodate new arrangements for the musical's score.

"This does require us to look again at the make-up of the orchestra to ensure we do credit to the score," said Mackintosh. "I hope that as many of them as possible will be able to remain with us for the next stage of the show's life. However, should personnel changes be required I must continue, as always, to put the show and audience first."

Mackintosh's assurance that he wanted to keep the Les Misérables' London show "the world's premiere production" have done little to appease its musicians. "I don't think what they've done is the right thing to do," said the oboist Adrian Rowlands, when contacted by The Independent. A flautist who has played on a Les Misérables national tour also confirmed the news, but declined to go on the record as he intended to apply for one of the jobs.

When the London show moved from the Palace Theatre to the Queens Theatre in 2004, the orchestra was cut to 11 members. A new, more complicated score, which involves "doubling" and "trebling" – the number of instruments each musician is required to play – was developed for a recent British tour, and the musical's composers, Alain Boubil and Claude-Michel Schönberg, hope to accommodate this into the permanent production. "After an unprecedented run of 25 years it is remarkable to think that we still have some members of the original orchestra working on the show," they told The Stage.

"The renewed popularity of the show in recent years has enabled Cameron to consider investment both in the new sound design and increasing the band to 14 in order to use the new orchestrations that we developed."

The show, based on the Victor Hugo novel of the same name, was applauded for its 25th anniversary concert last October. The performance featured a company of 300 actors and musicians.

"Les Misérables has been playing around the world for 25 years since it opened at the Barbican in 1985," concluded Mackintosh. "Such longevity would not have been possible had the show not constantly evolved and always been presented to the highest possible standard. With the help of my creative teams, we polish, change and improve elements of the production all the time to ensure that the show continues to enthral and entertain audiences."